The present invention relates to a method and device for determining several parameters of a person sitting on a seat, such as, for example, the size and/or the weight of the passenger and/or the orientation of the passenger on the seat. Such a device is particularly applicable in the area covering the control of the protection system in motor vehicles.
In order to protect the lives of passengers during a traffic accident, modern vehicles are generally fitted with a protection system comprising several airbags and seat belt pre-tensioners, which are used to absorb the energy of a passenger released during the collision due to the accident. It is clear that such systems are even more effective when they are better adapted to the specific requirements of each passenger, i.e. to the weight and/or the size of the passenger. That is why microprocessor-controlled protection systems have been designed which provide several operational modes, for example allowing an adaptation of the instant at which airbags are deployed and their volume, of the instant at which safety belts are released after the collision, etc, as a function of the stature of the passenger and the orientation of the passenger on the seat.
In order to enable the control microprocessor to select the optimum operational mode for a given passenger, it is therefore necessary to have available a device for detecting the stature of the passenger which determines the size and/or the weight and/or the orientation of the passenger and which indicates this to the control circuit of the protection system. For this purpose, the U.S. Pat. No. 5,232,243 describes a device for detecting the weight of a passenger which comprises several individual force sensors arranged in a matrix array in the vehicle seat cushion. The force sensors have an electric resistance that varies with the applied force and are known by the abbreviation FSR (force sensing resistor). The resistance of each sensor is measured individually and, by adding the forces corresponding to the values of these resistances, an indication is obtained of the total force exerted, i.e. of the weight of the passenger.
However, the total weight of a passenger does not act solely on the surface of the seat, since part of the weight is supported by the passenger""s legs, which rest on the bottom of the vehicle, and another part rests on the back of the seat. In addition, the ratios between the various parts vary considerably with the passenger""s position on the seat, which causes the total force measured by the individual force sensors not to correspond to the real weight of the passenger but to experience very large variations depending on the passenger""s posture on the seat.
Moreover, because of variations with temperature of the characteristics of the seat padding, the individual forces measured by the different force sensors depend greatly on the ambient temperature in the vehicle. In fact, at very low temperatures, foam padding for example becomes very hard, causing the forces measured by the sensors to be less than the real forces. At very high temperatures, on the other hand, foam padding expands and exerts an additional pressure on the sensors, so that the forces measured by the sensors are greater than the forces actually exerted. It follows from this that the device for detecting the weight of a passenger, as described in the above-mentioned document, cannot really satisfy the requirements of a modern protection system whose operation must to a large extent be independent of the ambient conditions.
The objective of the present invention is therefore to propose a device for determining several parameters of a person sitting on a seat, the operation of which is to a large extent independent of the temperature and of the passenger""s posture on the said seat.
In conformity with the invention, this objective is achieved by a device making it possible to determine the size and/or the weight of a person sitting on a seat, which operates according to a principle different from that of existing weight detectors. The method used for determining the size and/or the weight of a person sitting on a seat involves the subdivision of the seat""s surface into at least two sections, the determination of the position of the centre of gravity of the active weight in each section, and the evaluation of the size and/or the weight of the said person from the said positions so determined. Measurements are therefore no longer made of the magnitude of the force exerted by the passenger on the seat, but measurements are instead made of the positions at which this force acts. In other words, relative values are now to be measured instead of absolute values. The positions at which the force acts determined in this way are therefore to a large extent independent of the factors affecting the absolute values of the force, such as the posture of the passenger on the seat and the ambient temperature. The respective positions of the centres of gravity in the different sections of the seat then make it possible to determine the size and/or the weight of the person and, for example, the person""s position and/or orientation on the seat.
By subdividing the seat, for example into a plurality of adjacent sections, and by determining the positions at which the weight acts in each of these sections, it is possible to determine the total area over which the weight is active, i.e. the area of the seat occupied by the passenger. Moreover, it is easy to determine the position of the passenger on the seat from the distribution of the different positions of the centres of gravity, and this makes it possible to assess whether the passenger is sitting in the middle of the seat. By comparing the longitudinal positions of the centres of gravity in different laterally adjacent sections of the seat, it is possible to determine the orientation of the passenger on the seat, i.e. whether the passenger is facing the front or a different direction. It should be noted that the different parameters are preferably assessed sequentially using the same detector.
In a preferred version of the method, the surface of the seat is subdivided into two laterally adjacent sections and the evaluation of the size and/or the weight involves determining the distance between the positions of the two centres of gravity of the weight in the said two sections. The parameter so determined is therefore the lateral distance between the position at which the weight acts on the left-hand part of the seat and the position at which the weight acts on the right-hand part of the seat, i.e. a distance which is correlated with the stature of the passenger. From this distance, it is thus possible to evaluate the weight and/or the size of the passenger by using a model of a human body based on statistical measurements.
It is true that a method of determination using a model of a human body cannot provide an exact measurement of the real weight of the seat""s occupant. However, in view of the restricted number (3 for example) of ways in which the airbags or seat belt pre-tensioners in a vehicle can function, the requirements for the control device of the protection system as regards the accuracy of the real value of the weight are only of secondary importance. It is in fact necessary only to allocate the different passengers to a restricted number of categories as regards weight and size for the control device to be able to select the appropriate operational mode to be applied. In the example of three operational modes for the protection system, three categories of weights have to cover a total range from, for example, 0 to 100 kg, i.e. each category must cover a range of about 30 kg. Now it is clear that, for a classification into such broad categories, the results obtained by evaluating weight and/or size using a human model to a great extent satisfy the requirements of accuracy in the system.
In order to work according to the method described above, a device for determining the size and/or the weight of a person sitting on a seat therefore comprises a means for determining the respective positions of the centres of gravity of the active weight in at least two different sections of the said seat and a means of evaluating the size and/or the weight of the said person from the said determined positions. The positions of the centres of gravity determined in this way give, for example, an indication of the total area over which the weight is active, i.e. the area of the seat which is occupied by the passenger. To achieve this, the surface of the seat should be subdivided into a large number of sections. However, in a preferred execution, the said means of determining the positions of the centres of gravity comprise a means of detecting the distance between a first centre of gravity of the weight on a first section of the seat and a second centre of gravity of the weight on a second section of the seat, the two sections of the seat being laterally adjacent. In other words, the lateral distance is measured between the position at which the weight acts, for example on the left-hand part of the seat, and the position at which the weight acts on the right-hand part of the seat, i.e. a distance which is related to the width of the area of the seat occupied by the passenger. This distance then makes it possible to evaluate the weight and/or the size in the way described above.
The means for determining the positions of the centres of gravity preferably comprise a position-defining force detector extending over the surface of the seat. Such a detector consists, for example, of a plurality of switching elements arranged in a plurality of adjacent sections of the seat. These switching elements are then interconnected in an nxc3x97m matrix array so that they can be individually identified. However, such a detector requires a large number (xe2x89xa7n*m) of connections with the outside, i.e. with the control device for the protection system, and inside the control device it requires a sophisticated electronic system for the real-time exploitation of the n*m signals from the different switching elements.
In an advantageous implementation, the said position-defining force detector comprises several active areas in the form of strips, the said active areas being positioned on both sides of a line separating the said two sections and extending parallel to it. The strip-shaped active areas then advantageously extend over a major part of the length of the seat""s surface, so that a determination of the width of the area occupied by passengers is independent of their longitudinal position on the seat. This implementation on the one hand considerably reduces the number of connections of the detector with the outside and on the other hand enables a less sophisticated electronic system to be used for the real-time exploitation of the signals from the active areas.
Advantageously, the said force detector comprises force sensors whose electric resistance varies with the applied force. These force sensors are known by the abbreviation FSR (force sensing resistors) and enable the value of the force applied to the active area to be detected directly. This direct measurement of the applied force thus enables the device according to the invention also to operate as a detector of the occupation of the seat. In other words, below a certain value of the force measured by the FSRs corresponding to a certain minimum weight acting on the seat, the protection system for the seat in question is not activated at all. During a collision due to an accident, a determination of the passenger""s weight category is made and the protection system is activated only if the limiting value of the force is exceeded.
For safety reasons, the device advantageously comprises a circuit for monitoring the integrity of the conductors. This circuit monitors the integrity of the conductors, for example when the vehicle starts up, and indicates to the control device of the protection system any breakdown in a connection or a conductor. In the case of such a breakdown which risks affecting the correct operation of the detection device, the control device will select a standard operational mode of the protection system which represents a compromise solution for all the weight categories.